A rare-earth alloy sintered magnet (permanent magnet) is normally produced by compacting a powder of a rare-earth alloy, sintering the resultant powder compact and then subjecting the sintered body to an aging treatment. Permanent magnets currently used extensively in various applications include rare-earth-cobalt based magnets and rare-earth-iron-boron based magnets. Among other things, the rare-earth-iron-boron based magnets (which will be referred to herein as “R—Fe—B based magnets”, where R is one of the rare-earth elements including Y, Fe is iron, and B is boron) are used more and more often in various electronic appliances. This is because an R—Fe—B based magnet exhibits a maximum energy product, which is higher than any of various other types of magnets, and yet is relatively inexpensive.
An R—Fe—B based sintered magnet includes a main phase consisting essentially of a tetragonal R2Fe14B compound, an R-rich phase including Nd, for example, and a B-rich phase. In the R—Fe—B based sintered magnet, a portion of Fe may be replaced with a transition metal such as Co or Ni and a portion of boron (B) may be replaced with carbon (C). An R—Fe—B based sintered magnet, to which the present invention is applicable effectively, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, for example.
In the prior art, an R—Fe—B based alloy has been prepared as a material for such a magnet by an ingot casting process. In an ingot casting process, normally, rare-earth metal, electrolytic iron and ferroboron alloy as respective start materials are melted by an induction heating process, and then the melt obtained in this manner is cooled relatively slowly in a casting mold, thereby preparing an alloy ingot.
Recently, a rapid cooling process such as a strip casting process or a centrifugal casting process has attracted much attention in the art. In a rapid cooling process, a molten alloy is brought into contact with, and relatively rapidly cooled by, a single chill roller, a twin chill roller, a rotating disk or the inner surface of a rotating cylindrical casting mold, thereby making a solidified alloy, which is thinner than an alloy ingot, from the molten alloy. The solidified alloy prepared in this manner will be referred to herein as an “alloy flake”. The alloy flake produced by such a rapid cooling process usually has a thickness of about 0.03 mm to about 10 mm. According to the rapid cooling process, the molten alloy starts to be solidified from its surface that has been in contact with the surface of the chill roller. That surface of the molten alloy will be referred to herein as a “roller contact surface”. Thus, in the rapid cooling process, columnar crystals grow in the thickness direction from the roller contact surface. As a result, the rapidly solidified alloy, made by a strip casting process or any other rapid cooling process, has a structure including an R2Fe14B crystalline phase and an R-rich phase. The R2Fe14B crystalline phase usually has a minor-axis size of about 0.1 μm to about 100 μm and a major-axis size of about 5 μm to about 500 μm. On the other hand, the R-rich phase, which is a non-magnetic phase including a rare-earth element R at a relatively high concentration and having a thickness (corresponding to the width of the grain boundary) of about 10 μm or less, is dispersed on the grain boundary between the R2Fe14B crystalline phases.
Compared to an alloy made by the conventional ingot casting process or die casting process (such an alloy will be referred to herein as an “ingot alloy”), the rapidly solidified alloy has been quenched in a shorter time (i.e., at a cooling rate of 102° C./s to 104° C./s). Accordingly, the rapidly solidified alloy has a finer structure and a smaller crystal grain size. In addition, in the rapidly solidified alloy, the grain boundary thereof has a greater area and the R-rich phase is dispersed broadly and thinly over the grain boundary. Thus, the rapidly solidified alloy also excels in the dispersiveness of the R-rich phase. Because the rapidly solidified alloy has the above-described advantageous features, a magnet with excellent magnetic properties can be made from the rapidly solidified alloy.
An alternative alloy preparation method called “Ca reduction process (or reduction/diffusion process)” is also known in the art. This process includes the processing and manufacturing steps of: adding metal calcium (Ca) and calcium chloride (CaCl) to either the mixture of at least one rare-earth oxide, iron powder, pure boron powder and at least one of ferroboron powder and boron oxide at a predetermined ratio or a mixture including an alloy powder or mixed oxide of these constituent elements at a predetermined ratio; subjecting the resultant mixture to a reduction/diffusion treatment within an inert atmosphere; diluting the reactant obtained to make a slurry; and then treating the slurry with water. In this manner, a solid of an R—Fe—B based alloy can be obtained.
It should be noted that any small block of a solid alloy will be referred to herein as an “alloy block”. The “alloy block” may be any of various forms of solid alloys that include not only solidified alloys obtained by cooling a melt of a material alloy (e.g., an alloy ingot prepared by the conventional ingot casting process or an alloy flake prepared by a rapid cooling process such as a strip casting process) but also a solid alloy obtained by the Ca reduction process.
An alloy powder to be compacted is obtained by performing the processing steps of: coarsely pulverizing an alloy block in any of these forms by a hydrogen occlusion process, for example, and/or any of various mechanical milling processes (e.g., using a disk mill); and finely pulverizing the resultant coarse powder (with a mean particle size of 10 μm to 500 μm) by a dry milling process using a jet mill, for example.
The R—Fe—B based alloy powder to be compacted preferably has a mean particle size of 1.5 μm to about 6 μm to achieve sufficient magnetic properties. It should be noted that the “mean particle size” of a powder refers to herein an FSSS particle size unless stated otherwise. However, when a powder with such a small mean particle size is used, the resultant flowability, compactability (including cavity fill density and compressibility) and productivity will be bad.
To overcome this problem, a method of coating the surface of alloy powder particles with a lubricant was proposed. For example, Japanese Laid-Open Publication No. 08-111308 and U.S. Pat. No. 5,666,635 disclose the technique of making an R—Fe—B based alloy fine powder (with a mean particle size of 1.5 μm to 5 μm) by adding 0.02 mass % to 5.0 mass % of a lubricant (including at least one liquefied fatty acid ester) to an R—Fe—B based alloy coarse powder with a mean particle size of 10 μm to 500 μm and then pulverizing the mixture by a jet mill within an inert gas.
The lubricant not only improves the flowability and compactability (or compressibility) of the powder but also functions as a binder for increasing the hardness (or strength) of the compact. Nevertheless, the lubricant may also remain as residual carbon in the sintered body to possibly deteriorate the magnetic properties. Accordingly, the lubricant needs to exhibit good binder removability. For example, Japanese Laid-Open Publication No. 2000-306753 discloses, as preferred lubricants with good binder removability, depolymerized polymers, mixtures of a depolymerized polymer and a hydrocarbon solvent, and mixtures of a depolymerized polymer, a low-viscosity mineral oil and a hydrocarbon solvent.
According to this method using a lubricant, however, a certain degree of improvement is achieved but it is still difficult to fill the cavity with the powder sufficiently uniformly or achieve a sufficient degree of compactability. Among other things, a powder made by a strip casting process or any other rapid quenching process (at a cooling rate of 102° C./s to 104° C./s) has a smaller mean particle size and a sharper particle size distribution than a powder made by an ingot casting process, and therefore, exhibits particularly bad flowability. For that reason, the amount of the powder to be loaded into the cavity may sometimes go beyond its allowable range or the in-cavity fill density may become non-uniform. As a result, the variations in the mass or dimensions of the compacts may exceed their allowable ranges or the compacts may crack or chip.
As another method for improving the flowability and compactability of an R—Fe—B based alloy powder, there was a proposal to make a granulated powder.
For example, Japanese Laid-Open Publication No. 63-237402 discloses that the compactability should be improvable with a granulated powder to be obtained by adding 0.4 mass % to 4.0 mass % of mixture of a paraffin compound (which is liquid at room temperature) and an aliphatic carboxylate to the powder, and mulling and granulating them together. A method in which polyvinyl alcohol (PVA) is used as a granulating agent is also known. It should be noted that the granulating agent, as well as a lubricant, functions as a binder for increasing the strength of the compact.
If the granulating agent disclosed in Japanese Laid-Open Publication No. 63-237402 is used, however, then the binder removability is so bad that the magnetic properties of an R—Fe—B based sintered magnet will be deteriorated by carbon remaining in the sintered body.
On the other hand, the granulated powder produced by applying a spray dryer method to PVA has high binding force and therefore is too hard to be broken completely even on the application of an external magnetic field. Accordingly, the primary particles thereof cannot be aligned with the magnetic field sufficiently and no magnets with excellent magnetic properties can be obtained. PVA also has bad binder removability and carbon derived from PVA is likely to remain in the magnets. This problem may be overcome by performing a binder removal process within a hydrogen atmosphere. However, it is still difficult to remove that carbon sufficiently.
To solve the problem that the granulated powder is difficult to break even under the aligning magnetic field, the applicant of the present application proposed a method of making a granulated powder, in which respective powder particles (i.e., primary particles) aligned with a magnetic field applied are coupled together with a granulating agent, by granulating the material powder with a static magnetic field applied thereto (see Japanese Laid-Open Publication No. 10-140202). If this granulated powder is used, the magnetic properties are improvable compared with using a granulated powder in which primary particles not aligned with a magnetic field applied are coupled together with a granulating agent. However, it is difficult to align the powder particles being pressed with the magnetic field sufficiently. Consequently, the resultant magnetic properties are lower than a situation where a non-granulated rare-earth alloy powder was used.
Various granulating agents and granulating methods have been proposed so far as described above. However, a method for mass-producing a rare-earth alloy granulated powder, which has excellent flowability and compactability and which can contribute to producing magnets with good magnetic properties, has not yet been developed.
On the other hand, demands for smaller, thinner and performance-enhanced magnets have been escalating. Thus, the development of a method for producing small or thin high-performance magnets with high productivity is awaited. Generally speaking, if a rare-earth alloy sintered body (or a magnet obtained by magnetizing the sintered body) is machined, then its magnetic properties will deteriorate due to a strain caused by the machining process. Such deterioration in magnetic properties is non-negligible in a small magnet. Accordingly, the smaller the size of the magnet to be obtained, the more necessary it is to make a sintered body that has so high dimensional accuracy as to need almost no machining at all and also has the final shape to be obtained. Demands for a rare-earth alloy powder with excellent flowability and compactability (e.g., an R—Fe—B based alloy powder among other things) have been further growing for these reasons, too.